1. Field of the Invention
The Present invention concerns transmission via optical fibers, to be more specific transmission via photonic crystal fibers.
2. Description of the Prior Art
The index profile of an optical fiber is generally qualified as a function of the shape of the graph of the function that associates the radius of the fiber and the refractive index. It is conventional to plot the distance r from the center of the fiber on the horizontal axis and the difference between the refractive index of the cladding and the refractive index of the fiber on the vertical axis. Thus the expressions xe2x80x9cstep index profilexe2x80x9d, xe2x80x9ctrapezium index profilexe2x80x9d and xe2x80x9ctriangle index profilexe2x80x9d are used with reference to graphs that are respectively step-shaped, trapezium-shaped and triangular. The curves are generally representative of a theoretical or set point profile of the fiber. Fiber fabrication constraints can yield a significantly different profile. Variations in the index in accordance with the profile control the propagation of light along the fiber.
Photonic crystal fibers have recently been introduced. Unlike conventional fibers, photonic crystal fibers do not consist entirely of a solid transparent material such as doped silica; seen in section, a photonic fiber has a plurality of holes filled with air. The holes are parallel to the axis of the fiber and extend longitudinally along the fiber. In practice, the holes can be obtained by fabricating the preform by assembling silica capillary tubes and cylinders in accordance with the pattern of the holes to be obtained in the fiber. Drawing this kind of preform produces a fiber with holes corresponding to the capillary tubes.
The holes in the material of the fiber create variations in the mean index of the material; as in a conventional optical fiber, these variations in the index can be used to guide optical signals at appropriate wavelengths. Photonic crystal fibers are described in WO-A-00 49 435: in addition to describing the theory of operation of photonic crystal fibers, the above document describes a method of assembling them. In cross section, the patterns of holes proposed in the above document are based on a matrix of triangular holes, i.e. the potential locations of holes form lines in three directions inclined at 60xc2x0 to each other. The omission of some holes in the matrix guides light; to be more specific, in one embodiment, the hole at the center of the fiber is omitted so that, in cross section, the fiber consists of a solid core surrounded by holes formed in accordance with the triangular matrix. In a second embodiment there are seven holes at the center of the fiber, at the vertices and the center of a regular hexagon. Holes are disposed around the central hexagon at the vertices of hexagons forming a mosaic across the cross section of the fiber; there is no hole at the center of the hexagons other than the central hexagon. The above document further proposes to use holes of different diameter, destroying the symmetry of the fiber on rotation through 60xc2x0 about its center; the object of this is to modify the birefringence of the fiber.
R. F. Cregan et al., xe2x80x9cDistribution of Spontaneous Emission from an Er3-Doped Photonic Crystal Fiberxe2x80x9d, Journal of Lightwave Technology, vol. 17, No. 11, November 1999 discusses spontaneous emission in a photonic crystal fiber. The holes filled with air are distributed in a triangular matrix and the fiber is hexagonal; there is no hole in the fiber at the center of the hexagon, and the silica is doped with erbium. The above document discusses the spatial distribution of the spontaneous emission when the fiber is pumped axially, and shows that this distribution is a function of the distribution of the holes in the fiber, which agrees with simulation results. No mention is made of any use of the doped fiber.
Thomas Sondergaard, xe2x80x9cPhotonic Crystal Distributed Feedback Fiber Lasers with Bragg Gratingsxe2x80x9d, Journal of Lightwave Technology, vol. 18, No. 4, April 2000 discusses the use of photonic crystal fibers to produce fiber lasers; the above document specifies that the mode surfaces for the signal or for the pump can be smaller or larger than the corresponding mode surfaces of conventional stepped index fibers. Photonic crystal fibers can therefore be used to produce fiber lasers with a low pumping thresholdxe2x80x94in the case of weak mode surfacesxe2x80x94or high-power lasersxe2x80x94in the case of strong mode surfaces. The above document refers only to digital simulation results, and ignores any practical implementation.
W J. Wadsworth et al., xe2x80x9cYb3+-doped photonic crystal fibre laserxe2x80x9d, Electronics Letters, vol. 36, No. 17, August 2000 describes the experimental achievement of a laser effect in a photonic crystal fiber; the fiber is made by surrounding pure silica capillary tubes with a silica tube doped with Yb and codoped with Al; this combination is then drawn to form a fiber, around which a pure silica sleeve is placed. Two rows of holes surround the core and light is strongly confined within the doped core of the fiber.
EP-A-1 043 816 describes a double-cladding fiber; the signal is transmitted in the doped core of the fiber and a pump is injected into the first cladding. To direct the light from the pump to the doped core, it is proposed to provide modified index regions in the first cladding. The modified index regions can in particular be holes filled with air. One embodiment includes three modified index regions distributed over the periphery of the first cladding. Another embodiment includes six modified index regions, forming the vertices and the mid-points of the sides of an equilateral triangle. It is suggested that the modified index regions must be placed as far as possible from the core of the fiber to avoid modifying the polarization in the core of the fiber.
The problem addressed by the invention is that of distributing the holes in a photonic crystal fiber to improve the effect of confinement of light in the fiber. The invention proposes a double-cladding fiber in which holes are provided to improve the overlap between the signal and the pump.
To be more precise, the invention proposes a photonic crystal fiber having a plurality of holes distributed over points of a regular matrix and wherein:
the holes are on at least two layers of points of the matrix concentric with the fiber,
the holes in a layer have the same dimensions and occupy all points of the layer, and
the holes in at least one layer have dimensions greater than or equal to the dimensions of the holes in another layer inside it.
In a preferred embodiment the matrix is a triangular matrix and the layers are hexagonal.
The fiber preferably has an effective surface area greater than or equal to 150 xcexcm2. The invention further provides a transmission system including the above fiber as line fiber, an optical amplifier including a section of the above fiber doped with at least one rare earth ion, and a fiber laser including a section of the above fiber.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention, which description is given by way of example and with reference to the accompanying drawings.